This invention relates generally to heat pumps having reversible vapor compression refrigerant systems and more specifically to automatic control of the defrost operation of the refrigerant system whereby successive defrost cycle operations are spaced apart variably in time as a function of the outdoor climate conditions to which the outdoor coil of the heat pump is exposed.
As is well known, frost build-up occurs on the outdoor coil of heat pump refrigerant systems when operating in the heating mode and it is necessary to periodically reverse the refrigerant flow in the system to heat the coil and thereby remove the frost.
Typically, heat pumps known in the prior art are caused to enter into the defrost cycle at regular fixed intervals of time. A preselected clocked time interval is provided which is established largely as the result of experience with heat pump operation in average winter climate conditions encountered in the field. The difficulty with this is that under extreme conditions of rapid frost build-up, the heat pump operates at reduced heating efficiency during heavy frost conditions on the outdoor coil before the system enters into defrost. Conversely, during very mild frost build-up conditions, the system is often caused to enter defrost operation prematurely since insufficient frost has formed to adversely affect the heating efficiency of the heat pump. In this case, however, even though the heat pump efficiency remains unchanged, the overall system efficiency is adversely affected. Defrost operation is the same as the cooling operating mode for the heat pump and it is, therefore, necessary to employ auxiliary heat sources, such as electrical strip heaters, to counteract the cooling effect that is imposed on the room space during the defrost cycle. It is this added use of power for heating during unnecessary defrost cycles that reduces the overall heating system operating efficiency from what would be possible with fewer defrost cycles, i.e. providing longer time intervals between defrost cycles under light frost build-up conditions. It is, therefore, desirable to provide means for altering the intervals between which the defrost cycle is initiated as a function of actual climate conditions in order to optimize the overall operating efficiency of the heating system of which the heat pump is a part.
It is, therefore, an object of the invention to provide a heat pump with automatic defrost control means adapted for adjusting the time between successive defrost operations as a function of the outdoor climate conditions to which the outdoor coil of the heat pump is exposed.
It is a further object of the invention to provide automatic defrost control for a heat pump that varies the time interval between defrost cycles as a function of variations in outdoor ambient temperature.
It is yet another object of the invention to provide a heat pump with defrost control means which permits alternative defrost cycle time vs. temperature functions to be employed, such functions having provision for relating the temperature to average climate conditions prevalent in respectively different geographical locations in which the heat pump may be installed. The desired function may be selected in accordance with a feature of the invention at the time of manufacture, installation or maintenance of the heat pump.